The present invention relates to liquid coating methods and apparatus in general, and more particularly to a method and apparatus for premetered "patch" coating discrete, incremental surfaces or substrates, such as printed circuit boards, integrated circuits, etc., wherein a premetered volume per unit area of liquid is applied to a surface with a liquid layer "patch" having a pre-configured shape and a predetermined coating thickness.
In the manufacture of printed circuit boards or integrated circuits it is often desirable to apply a liquid coating to the surface of the circuit board or IC wafer, and it is critical that the thickness of that wet coating be uniform across the surface of the board or substrate. In the case of photoresists or photo-imagable solder masks which are solidified either by drying or by crosslinking with radiation, the thickness of the final solidified coating will generally be as uniform as that of the wet coating applied.
In manufacturing printed circuit boards, a photoresist is used to transfer the outline of the circuit into the copper surface of the board. The term "photoresist" defines the dual functioning nature of the material. First it is a photo polymer whose chemical properties are changed by exposure to ultraviolet radiation. That exposure is done selectively through a mask outlining the circuit being defined. The dual functioning comes into play after developing the imaged photo polymer, where the soft unwanted areas are washed off the copper surface. What remains is a protective covering of hardened polymer only in those areas outlined by the exposure mask. In one application, the circuit board is then exposed to an etchant. The protective covering resists the etching process so that only the copper surface left unprotected is etched away. When the developed resist is finally chemically stripped away, the protected copper circuit lines underneath become the electrical conductors of the circuit board.
In the current manufacture of printed circuit boards, the chemical processes of developing the imaged resist, etching the exposed copper, and stripping off the developed resist are all accomplished by exposing the circuit board to a substantial oversupply of the developer, etchant or chemical stripper. This is usually done through a spray process that also provides agitation to enhance removal of the softened or dissolved surface material. These processes are costly, cumbersome and inefficient because by recycling the chemicals for economic reasons, the circuit board's surface is exposed to previously reacted material. This reduces its chemical efficiency in reacting with the surface, increasing cycle time and reducing circuit definition. In addition it is difficult to maintain the quality of the recycled material which causes its rate of reaction with the surface to vary.
It is, accordingly, a general object of the invention to provide an improved coating method and apparatus.
It is a specific object of the invention to provide a method and apparatus for premetered "patch" coating discrete, incremental coating surfaces or substrates in which a controlled volume per unit area of a liquid is applied to the surface or substrate with a liquid layer "patch" having a pre-configured shape and coating thickness.
It is another object of the invention to provide a method and apparatus for premetered "patch" coating discrete, incremental coating surfaces or substrates in which controlled volumes per unit area of liquids are applied together in superposed relation to the surface or substrate with each superposed liquid layer "patch" having a pre-configured shape and coating thickness.
It is a feature of the invention that the coating method can be employed to coat various printed circuit board process liquids on printed circuit boards without requiring a substantial oversupply of each process liquid.
It is another feature of the invention that the apparatus provides for the controlled application of liquids from a sharply defined start to a sharply defined stop of the liquid coating or coatings applied to the substrate surface.